Recently, as the Internet and various data services are rapidly increased, a large amount of transmission capacity is needed in a subscriber network, and a local area network (LAN). In order to comply with such requirement, an optical access network and a local area network have attracted considerable attention. Here, the optical subscriber network is constructed on the basis of single mode optical fibers which are connected to houses to provide services thereto, which is called Fiber-To-The-Home (FTTH). Also, the local area network based on a multi-mode optical fiber improves performance of the prior art multi-mode optical fiber, and can transmit a fast signal of 10 Gb/s to a distance of 300 m using an electrical compensation device and an offset launching technology of a light source.
FIG. 1(a) and FIG. 1(b) are refractive index profiles of a single mode optical fiber and a multi-mode optical fiber used in the prior art optical communication network, respectively.
In FIG. 1(a) and FIG. 1(b), n1 denotes the maximum refractive index in a core center, n2 denotes a refractive index of a cladding, and n0 denotes the refractive index of air. Also, a shows a core radius, and b shows a radius of an optical fiber including a core and a cladding.
The optical access network based on the FTTH, which employs a single mode optical fiber of FIG. 1(a), is superior to an xDSL network using copper wiring, regarding transmission capacity and transmission quality, however, it has disadvantages in that, since it utilizes relatively expensive optical components, it is not cost-effective. Namely, since the core diameter of the prior art single mode optical fiber is relatively very small, about 8 μm, it requires relatively much packaging cost such that outputs of optical components can be effectively coupled to the optical fibers. Such packaging cost can be largely reduced, as a multi-mode optical fiber having a relatively large diameter, such as 50 μm or 62.5 μm, as shown in FIG. 1(b), is employed. However, when a multi-mode optical fiber is used, since there are a plurality of modes propagating through the optical fiber, and modal dispersion is large due to delay between modes, the prior art network has disadvantage in that ultra-high speed data over 10 Gb/s cannot be transmitted long distances.
Therefore, research into a network structure has been performed to reduce network construction costs based on the single mode-optical fiber in the prior art optical access network. Recently, research has been conducted into light sources for economically constructing a network and means for economically implementing components adopted in a network. Even though research into cost reduction has been conducting, it must be limited as long as prices for optical components is not reduced. On the other hand, an offset launching technology and various electrical compensation devices have been researched such that ultra-high speed data of 10 Gb/s based on a multi-mode optical fiber can be transmitted 300 m in the local area network, however, such endeavor is focused only on performance improvement of a transmission system without cost consideration.